同步线程执行

Question

我的任务是按以下顺序创建线程：如果A开始->启动B和C，如果B开始->启动D。并以相反的顺序销毁它们如果D然后B。如果B和C然后A。它。我设法做到了，但我想还有更好的方法。你有什么建议吗？

在您发表评论后，我更改了代码，这非常简单。但是现在看起来“愚蠢”。我想更改if语句和实现的硬性，有什么建议吗？寻求建议，我正在与您一起学习。

这是我的新代码：

     import java.util.*;
class RobotController implements Runnable{
String name;

public void run() {
    Thread t = Thread.currentThread();
    System.out.println(t.getName() + " status = " + t.isAlive());
    System.out.println(t.getName() + " status = " + t.getState());

  }

public static void main(String args[]) throws InterruptedException{
    Thread thread_A = new Thread(new RobotController(), "Thread A");
    Thread thread_B = new Thread(new RobotController(), "Thread B");
    Thread thread_C = new Thread(new RobotController(), "Thread C");
    Thread thread_D = new Thread(new RobotController(), "Thread D");

    thread_A.start();
    thread_A.join();
    System.out.println(thread_A.getState());
    thread_B.start();
    thread_B.join();
    System.out.println(thread_B.getState());
    thread_C.start();
    thread_C.join();
    System.out.println(thread_C.getState());
    thread_D.start();
    System.out.println(thread_D.getState());


}

}

Answer 1

您的代码中存在一些缺陷，这些缺陷有时会使它无法相应地工作：

1. 您打了电话thread_A.start()然后检查了thread_A.isAlive()。现在，如果thread_A.isAlive()条件检查之前thread_A已经完成，该怎么办？thread_B并且thread_C永远不会开始。您的应用程序失败。
2. 假设thread_A未完成且thread_A.isAlive()条件已通过，那么Java线程调度程序并不总是保证thread_Bbefore的启动thread_C。同样，您的应用程序失败。
3. 假设检查之前thread_B开始，thread_C并且如果检查thread_B完成之前thread_B.isAlive()，则if条件失败并且thread_D永远不会开始。同样，您的应用程序失败。

现在需要考虑的一点是：
在join()调用其方法之后，无需检查线程是否处于活动状态。这是不必要的运行时开销。

编辑
好，这是代码的修改版本。.我希望它可以让您了解线程的动态：

class RobotController implements Runnable
{
    private final Object lock = new Object();
    private void notifyThread()
    {
        synchronized(lock)
        {
            lock.notify();
        }
    }
    public void run() 
    {
        synchronized(lock)
        {
            try
            {
                System.out.println(Thread.currentThread().getName() + " started");
                lock.wait();
                System.out.println(Thread.currentThread().getName()+ " stopped");
            }
            catch (InterruptedException ex)
            {
                ex.printStackTrace();
            }
        }
    }

    public static void main(String args[]) throws InterruptedException
    {
        RobotController rca = new RobotController();
        RobotController rcb = new RobotController();
        RobotController rcc = new RobotController();
        RobotController rcd = new RobotController();


        Thread thread_A = new Thread(rca,"Thread A");
        Thread thread_B = new Thread(rcb,"Thread B");
        Thread thread_C = new Thread(rcc,"Thread C");
        Thread thread_D = new Thread(rcd,"Thread D");

        thread_A.start();
        while (thread_A.getState() != Thread.State.WAITING)
        {
            Thread.sleep(100);
        }
        thread_B.start();
        thread_C.start();
        while (thread_B.getState() != Thread.State.WAITING && thread_C.getState() != Thread.State.WAITING)
        {
            Thread.sleep(100);
        }
        thread_D.start();
        while (thread_D.getState() != Thread.State.WAITING)
        {
            Thread.sleep(100);
        }
        rcd.notifyThread();
        thread_D.join();
        rcc.notifyThread();
        thread_C.join();
        rcb.notifyThread();
        thread_B.join();
        rca.notifyThread();
    }

}

这是输出：

Thread A started
Thread B started
Thread C started
Thread D started
Thread D stopped
Thread C stopped
Thread B stopped
Thread A stopped